1. Field of the Invention
The invention relates to a mask for photolithography, and more particularly to a mask and method for defining a guard ring pattern.
2. Description of the Related Art
Memory devices of integrated circuits (ICs), such as a dynamic random access memory (DRAM) typically include a memory array region and a logic circuit region (or peripheral circuit region), wherein a guard ring is disposed around the logic circuit region to prevent electrostatic discharge or circuit noise from damaging the memory device.
Conventionally, the fabrication of the guard ring forms a ring region composed of a plurality of trenches in the substrate around the logic circuit region. Next, ion implantation is performed on those trenches to form a ring-shaped doping strap serving as a guard ring region. The trenches used for the fabrication of the guard ring are isolated, but those used for the fabrication of the logic circuit are dense. It is difficult to define all the trench patterns on the guard ring region when photolithography is performed to simultaneously define the guard ring and logic circuit regions.
FIG. 1 is a plane view of a conventional mask for defining a guard ring pattern. The mask includes a transparent substrate 10 and a light-shielding layer 12 thereon. The light-shielding layer 12, such as chromium, has a memory array pattern (not shown), a logic circuit pattern 16, and a plurality of rectangular opening patterns 14 around the logic circuit pattern 16. The logic circuit pattern 16 is composed of dense lines/spaces. Here, in order to simplify the diagram, only a blank block is shown. In addition, the rectangular opening patterns 14 define a guard ring pattern.
FIG. 2 is a plane view of a patterned photoresist layer formed by the mask in FIG. 1. A semiconductor substrate 20 is covered by a photoresist layer 21. After photolithography is performed on the photoresist layer 21 using the mask in FIG. 1, the patterns 16, 14 are transferred to the photoresist layer 21, thereby forming a guard ring pattern composed of rectangular opening patterns 24 around a logic circuit pattern 26 inside.
Unfortunately, since received dose in photoresist layer 21 for dense pattern (logic circuit pattern 26) and isolated pattern (guard ring pattern) are different to induce optical proximity effect (OPE), some opening patterns 14 cannot transfer to the photoresist layer 21 after developing. As a result, non-pattern region 22 or pin-hole pattern 23 are formed therein instead of the opening pattern 24. That is, an incomplete guard ring structure is formed in the substrate 20, thus the protection function fails.